Fighting expanded in eastern Ukraine on Tuesday, a day ahead of planned peace talks, with both Ukrainian forces and pro-Russian rebels claiming advances as a key government-held town far behind the front line was shelled for the first time in months.

Both sides appeared to jostle for position ahead of negotiations among the leaders of Ukraine, Russia, Germany and France set for Wednesday in Minsk, Belarus.

With much riding on the summit, President Obama on Tuesday called his counterparts in Russia and Ukraine to emphasize U.S. support for Ukraine’s sovereignty while urging Russia to stand by its commitments to a previous cease-fire.

Meanwhile, European Union leaders plan to meet this week to discuss new sanctions against Russia for allegedly aiding the rebels.

Efforts to impose a new cease-fire have coincided in recent weeks with escalating violence in eastern Ukraine, where the rebels have advanced against government forces after months of relative quiet. More than 5,300 people have died, and more than a million have been displaced from their homes, according to United Nations estimates.

With world leaders striving to find a solution to the crisis in Ukraine, rocket attacks in Kramatorsk and Luhansk are driving a rebel recruitment drive in Donetsk. (Reuters)

Ukraine and its Western allies have accused Russia of saying it desires peace in Ukraine while fueling the conflict with arms and fighters. Russia denies sending arms to the rebels, and a top Russian security official said on Tuesday that his country favored a united Ukraine.

“The Russian Federation definitely favors the territorial integrity of Ukraine within existing borders. Providing that Kiev is ready to heed and take into account the opinion of its regions and various groups of citizens,” Nikolai Patrushev, the head of Russia’s Security Council, said in an interview with the official Rossiyskaya Gazeta daily, according to an advance copy reviewed by the Interfax news service.

The diplomacy is focused on restoring a cease-fire signed in September but often violated since then. Rebels launched a full-on assault in early January and declared their intention to mobilize 100,000 fighters to conquer a far larger swath of eastern Ukraine.

On Tuesday, at least 15 people were killed and 63 wounded when rockets hit the government-held town of Kramatorsk, according to Oleksandr Turchynov, the head of Ukraine’s National Security and Defense Council. Kramatorsk is 30 miles behind the front line and has seen little fighting since July.

The Ukrainian military has been using the town as its eastern base. One fusillade hit the military headquarters, and another hit a residential neighborhood, Ukrainian President Petro Poroshenko told parliament. He said the attack had come from a Tornado multiple-rocket system.

Local news agencies published images of killed civilians, including one woman lying in the snow in a residential courtyard with a projectile embedded in the ground nearby. The images appeared to indicate the use of cluster munitions, according to Ole Solvang, a researcher with Human Rights Watch.

The Ukrainian military said on its Facebook page that the attack had probably come from Horlivka, a rebel-held town about 40 miles away. Rebels denied having fired the rockets.

Pro-government forces on Tuesday opened an assault against rebel positions near the government-held port city of Mariupol. The government-affiliated Azov Battalion wrote on a Russian-language social media page that it had deployed a “full-scale offensive” against rebel positions to push them away from the city. Last month, 30 civilians were killed in Mariupol when rockets fired from rebel-held territory hit an apartment building.

Ukrainian forces “broke through the enemy defenses,” the Ukrainian National Security and Defense Council said in a statement. Seven Ukrainian soldiers had been killed in the previous 24 hours, military spokesman Andriy Lysenko said.

In the meantime, Ukrainian and rebel forces said the government-held town of Debaltseve was nearly encircled by rebel fighters, raising the prospect of an imminent siege and large-scale Ukrainian losses.

“The Artemivsk-Debaltseve highway is blocked by terrorists,” Semyon Semenchenko, a Ukrainian militia commander and member of parliament, wrote on Facebook, using the government’s term for the rebels. “The roads are mined with land mines.”

Karoun Demirjian contributed to this report.